A lithium-ion secondary battery includes a positive electrode and a negative electrode that are separated by a porous separator. Lithium-ion secondary batteries are produced by use of a separator roll including a cylindrical core and such a separator wound around the core. A separator which is being produced may have a defect due to, for example, adhesion thereto of a foreign object. This necessitates inspecting the separator for a defect. Particularly in a case where a separator has a defect caused by an electrically conductive foreign object such as metal, the foreign object may cause a short circuit inside the lithium-ion secondary battery.
Patent Literature 1 discloses a defect inspection device configured to (i) emit visible light and infrared light to a surface of a sheet which is being transferred by a transfer roller and (ii) determine, in accordance with image captured data corresponding to the amount of each of the visible light reflected and the infrared light reflected, whether a defect on the surface of the sheet is caused by metal.